An email, usually very short, sent for the purpose of testing whether the email system is working. The sender's purpose may be:
- to find out whether the sender's outgoing email is configured correctly
- to troubleshoot the end-to-end connection between a sender and recipient

If the sender is able to find his solution with no response from the recipient, the test email will usually contain a terse instruction like "ignore this email."

If the sender needs a response from the recipient in order to complete the test, then the test email will say something like "let me know if you got this."